Ion mobility mass spectrometer

ABSTRACT

An ion mobility mass spectrometer for helium doped gases including a helium resonance lamp having an emission at 584A and a window in the lamp partially transparent to radiation at 584A. The window is contained within a channel through which the gases pass. A collector plate is placed opposite the window in the channel and means are provided for maintaining the window at a negative potential relative to the collector plate. When the lamp is excited by an RF source electrons and negative ion are collected on the exterior of the window and the positive ions travel to the collector plate. A monitoring device is connected to the collector plate.

United States Patent [1 1 Young Aug. 26, 1975 l l ION MOBILITY MASS SPECTROMETER [76] Inventor: Robert A. Young. R.R. No. 2,

Loretto Ontario, Canada [22] Filed: July 12, 1974 [21] Appl. No.: 488,188

OTHER PUBLICATIONS Poschcnrieder et a1. Gas Analysis by Photoionization Mass Spectrometry, Jour. Appl. Phy., Vol. 37, No 7. June 1966, pp. 2812-2819.

MONITOR Primary ExaminerJames W. Lawrence Assistant Examine'r-T. N. Grigsby Attorney, Agent, or Firm.lohn E. Benoit [57] ABSTRACT An ion mobility mass spectrometer for helium doped gases including a helium resonance lamp having an emission at 584A and a window in the lamp partially transparent to radiation at 584A. The window is contained within a channel through which the gases pass. A collector plate is placed opposite the window in the channel and means are provided for maintaining the window at a negative potential relative to the collector plate. When the lamp is excited by an RF source electrons and negative ion are collected on the exterior of the window and the positive ions travel to the collector plate. A monitoring device is connected to the collector plate.

4 Claims, 5 Drawing Figures PATENTED AUG 2 6 I975 SHEET 1 UP 2 SAMPLE SOURCE MONITOR 3802,06 PATE TEU AUG 2 61975 ION MOBILITY MASS SPECTROMETER This invention relates generally to mass spectrometers and more specifically to mass spectrometers using a helium resonance lamp.

The usual mass spectrometers require that the ion meanfree paths be large relative to the ion path lengths from ionization region to collector. Because of this, a gas sample must be greatly reduced in pressure before being ionized, and a compromise made between ionization efficiency (generally produced by an electron beam), extraction efficiency and collision free ion paths in the spectrometer. This requires elaborate pressure reduction systems involving high vacuum pumps and small orifices across which a large pressure drop occurs.

The present invention, on the contrary, operates using apparatus having dimensions which are large with respect to ion mean free path. Furthermore, this invention embodies an effective surface ionization source which is efficient at high pressures. These attributes result in a device which is much simpler, lighter, more sensitive, and directly applicable to measuring the composition of high pressure gas samples.

Accordingly, it is an object of this invention to provide an ion mobility mass spectrometer for measuring the composition of high pressure gas samples.

A further object of this invention is to provide an ion mobility mass spectrometer having an effective surface ionization source which is efficient at high pressures.

These and other objectsof the invention will become apparent from the following description with the drawings wherein FIG. 1 is a schematic illustration of the helium resonance lamp used in the present invention;

FIG. 2 is a schematic illustration of the mass spectrometer of the present invention;

FIGS. 3, 4a and 4b are graphical illustrations of the output of the device of FIG. 2.

Broadly speaking, the present invention provides an ion mobility mass spectrometer for helium doped gases including a helium resonance lamp having an emission at 584A and a window in the lamp partially transparent to radiation at 584A. The window is contained within a channel through which the gases pass. A collector plate is placed opposite the window in the channel and means are provided for maintaining the window at a negative potential relative to the collector plate. When the lamp is excited by an RF source electrons and negative ion are collected on the exterior of the window and the positive ions travel to the collector plate. A monitoring device is connected to the collector plate.

Turning now to FIG. 1 there is shown therein a preferred embodiment of a helium resonance lamp used in the present invention. The basic structure of the lamp is described in US. Patent Application Ser. No. 426,616 now US. Pat. No. 3,851,214 entitled Low Power Sealed Optically Thin Resonance Lamps, filed in the name of the present inventor.

Basically, the lamp 11 comprises a hollow cylindrical body 13 having a dielectric wall, such as glass, with a reentrant coaxial hollow glass element 17 located centrally within body 13. An electrical conductor 19 is connected to a source of RF energy 20. An integral arm 16 extends from cylindrical body 13 and contains a material 18 which acts as a getter such as uranium or barium. A gas permeable filter 22 such as glass frit maintains material 18 in position. Cylindrical body 13 is filled with high purity helium and a thin window 23, preferably of aluminum, is provided so as to pass only the desired radiation.

Window 23 is partially transparent to 584A radiation. The helium gas within cylindrical body 13 is maintained at a pressure between 0.1 and torr.

Thus, there is provided a helium resonance lamp having an emission at substantially 584A with a thin, i.e., 1000A, window partially transparent to 584A radiation. The window 23 is designed so as to be able to withstand high pressure when immersed in a gas mixture such as a mixture wherein one of the components is helium at a pressure between 0.1 and 100 torr.

The cylindrical body may be covered by an electrically conductive material 21 which is electrically grounded as is schematically shown. An example of a means for accomplishing this is when cylindrical lamp body 13 is enclosed within a close fitting conductive housing which is grounded. Therefore, the lamp body is effectively sheathed by a grounded conductive element. This element completes the necessary path for electrical excitation by RF source 20.

When lamp 11 is electronically excited by the RF source 20, helium radiation is passed by the window and adsorbed by He outside the lamp and, subsequently, this energy is transferred from the helium to other components of the gas mixture. This transfer may occur either directly, or through collisions of electrons, whose energy has been increased by superelastic interactions with excited helium: or as a consequence of ion neutralization (either with a free electron or with an attached electron in the form of a negative ion).

Of those materials which pass 584A, aluminum is preferred for practical reasons.

Turning now to FIG. 2 there is schematically illustrated therein the present invention using the helium resonance lamp ll of FIG. 1 driven by the RF source 20. Helium lamp 11 is mounted so that the aluminum window 23 is located inside a narrow channel 65 constructed of non-conductive material through which a helium doped gas sample is provided from source 64. The channel is relatively narrow so that window 23 is separated from collector plate 69 by between 1mm and 10cm.

Opposite the aluminum window 23 of the resonance tube 11 and in the channel wall is located a collector plate 69. The aluminum window 23 is maintained at a negative potential relative to the collector plate 69 by means such as a voltage source 67. The output of the collector plate 69 is supplied to the monitoring device 73 such as an electrometer including an input resistor 71.

In operation, the helium lamp 11 is pulse excited by the RF exciter 20 and so produces a thin sheath of ionization 63 in the helium doped gas sample 65 on the aluminum foil lamp window 23.

Since the aluminum foil window 23 is held at a negative potential relative to the collector plate, this applied electric field between the lamp window and the collector plate produces an action wherein the electrons (and negative ions) are collected on the aluminum window 23 while the positive ions drift toward the collector plate 69 with the velocity v; K,-KE where V, is the velocity of the ith ion species and K; is the mobility constant of the ith ion species and E is the uniform electric field between the window and collector. The lighter ions have the higher velocity and so arrive at the collector plate first. The current induced in the input resistor 71 of the electrometer 73 causes a voltage to appear across this resistor which is amplified and then recorded as shown in FIG. 3.

The lightest ions have the highest velocity and so arrive first at the collector plate 69. The induced current in resistor 71 is proportional to their total number of ions in the thin ion sheath, as well as to their velocity. The original sheath formed on the lamp window decomposes into many ion sheaths as the ions travel to the collector plate, each having a unique velocity. The sensitivity decreases with decreasing velocity and hence, high mass ions produce a smaller signal.

The distributionof ion travel time for the distance between the lamp window and collector plate occurs because K depends upon the ion mass such that the light ions travel faster than heavy ions. During this drift toward the collector the ion sheath spreads in thickness due to mutual repulsion of like charged ions and because of molecular diffusion. With proper adjustment of RF excited pulse width, electric field and lamp intensity, diffusion can be made to predominate in broadening the ion sheath. The shape of the decreasing output signal, if differentiated, represents'the number density distribution of ions in a direction perpendicular to the ion sheath. Proper choice of conditions can insure that the width of this distribution, on a time scale, is small with respect to the time taken to transit the windowcollector gap.

FIG. 3 represents the voltage across the input resistor vs time and illustrates the above operation. Input resistance voltage is plotted on the vertical axis and the time after formation of single sheath is plotted along the horizontal axis. The voltage drop indicated at a occurs when the lightest ion species arrives at the collector plate, and the drop marked b occurs when the next to heaviest ion species arrives at the plate.

FIG. 4a is an enlarged fragmentary view of one representative voltage drop and FIG. 4b is the derivative of the curve of FIG. 4a and shows the distribution of a single ion specie normal to the ion sheath.

The above description and drawings are illustrative only since equivalents could be substituted without departing from the invention. Accordingly, the invention is to be limited only the the scope of the following claims.

I claim:

1. An ion mobility mass spectrometer for helium doped gases comprising a helium resonance lamp having an emission at a window in said lamp partially transparent to radiation at 584A;

a channel of non-conductive material containing said window through which said gases pass;

a collector plate in said channel opposite said window;

means for maintaining said window at a negative potential relative to said collector plate; and

means for measuring an induced voltage resulting from ion movement within said gases between said window and said collector plate.

2. The spectrometer of claim 1 wherein said window is aluminum.

3. The spectrometer of claim 1 wherein said window and said collector plate are separated by a distance of between 1mm and 10cm.

4. The spectrometer of claim 1 further comprising means for monitoring said induced voltage. 

1. An ion mobility mass spectrometer for helium doped gases comprising a helium resonance lamp having an emission at 584A; a window in said lamp partially transparent to radiation at 584A; a channel of non-conductive material containing said window through which said gases pass; a collector plate in said channel opposite said window; means for maintaining said window at a negative potential relative to said collector plate; and means for measuring an induced voltage resulting from ion movement within said gases between said window and said collector plate.
 2. The spectrometer of claim 1 wherein said window is aluminum.
 3. The spectrometer of claim 1 wherein said window and said collector plate are separated by a distance of between 1mm and 10cm.
 4. The spectrometer of claim 1 further comprising means for monitoring said induced voltage. 